improvement(k8s-gke): clean up orphaned GKE disks
improvement(k8s-gke): clean up orphaned GKE disks

Without this change deleting each GKE cluster we leave orphaned
two following kinds of disks:
- 10Gb, created for MinIO service
- 10Gb, created for a Scylla which is dedicated for a Scylla Manager

These stay undeleted because it gets created dynamically and Google
doesn't delete it with deletion of a GKE cluster.
So, do it ourselves, and delete all detached GKE disks as part of the
clean up.

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests